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<A NAME="BABBEBBI"></A><h1>Distributed application architecture</h1>
<A NAME="TI3984"></A><p>Distributed application development, also called multitier
development, offers a natural way to separate the user interface
components of an application from the business logic that the application
requires. By centralizing business logic on a middle-tier server,
you can reduce the workload on the client and control access to
sensitive information.</p>
<A NAME="TI3985"></A><p>In a distributed application, the client and server work together
to perform tasks for the business user. The client handles all interactions
with the user while the middle-tier server provides background services
to the client. Typically, the middle-tier server performs most of
the processing and database access. To invoke the services of the
server, the client calls a method (or function) associated with
a component (or object) that resides on the server. </p>
<A NAME="TI3986"></A><h4>Partitioned applications</h4>
<A NAME="TI3987"></A><p>Client-side logic for enterprise applications must be as small
and efficient as possible to conserve network bandwidth. To accomplish
this goal, applications are partitioned into three parts: presentation,
business logic, and database access. The database resides on the
bottom tier of the enterprise system to maintain and secure the
organization's information assets. The business logic resides
in the middle tier or server. The presentation is on the user's
desktop, or top tier, or is dynamically downloaded to the user's
desktop.</p>
<A NAME="TI3988"></A><p>The server is then responsible for executing and securing
the vast majority of a corporation's business logic. This
makes it a critical component in the network-centric architecture.
The client communicates with the server, calling middle-tier components
that perform business logic. </p>
<A NAME="BABHHBFE"></A><h4>Web application architecture</h4>
<A NAME="TI3989"></A><p>A Web application is a variation of the distributed architecture
where the client is hosted in a Web browser. PowerBuilder provides
several technologies for building Web applications, including .NET
Web Forms deployment and the Web DataWindow, which provide a thin
client solution. The architecture of your application varies depending
on which technologies you decide to use.</p>
<A NAME="TI3990"></A><p>For more information, see <A HREF="apptechp191.htm#CAICDIBB">Chapter 29, "Web Application Development with
PowerBuilder."</A></p>

